Thought some here might like to see how I'm putting a full width Jag IRS into a '68 El Camino.

Made some actual progress!
Ideal had to yield to practical in a couple of respects.

Lowered rear ride height l.00" to only 8" to the frame bottom just ahead of the kick-up.
GOOD.
Lowers center of gravity for better handling.
Helps to get my engine to sit normally for it's designed angle.
Brings the lower trailing arms for bracing the bottom of the diff into an almost strait line.
Brought my main diff mount up on the frame rails.
BAD.
Increases the bend I need for the upper forward reaching diff mount reinforcements.
Reduced compression travel to only 3", maybe 3.5"

Here is an actual mock-up of my recent progress.
Main mount will be bushed at the ends.
Shock mounts and their spreaders are tacked in place.
Mounting points for the forward facing upper reinforcement are tacked in.
Lots of clearance for brake components.
Plan is adjustable shocks on the front and air shocks to level the ride when I have a load in the back.
The round tube behind the diff is a mock-up for possible reinforcement of the rear lower diff.
30* black bars are my fixed points for shock simulation at ride height.

I have to bend tubes for the forward reinforcements.
Despite my desire for light weight I seem to be overbuilding this at only 500 HP.
I hope the added weight at least helps to balance me out front to rear.

Looks pretty good Richard, a few notes:
Ideally the LCAs are installed with a slight downward angle from center to outside.
You will be adding tie bars front and rear for the pivot rods, yes? (These can be relatively thin.) And a very good idea is to include struts from the rear of the pivot rods up and out to your top bar to brace the housing against the torque reaction to the driveshaft.

Not made yet but a cross-braced pair of supports will extend forward from the main mount to the old upper trailing arm mounts in the frame.

Old lower trailing arm mounts will have arms to the bottom front diff tie-bar.
Rear diff tie-bar to vertical links joining a cross-bar behind the diff.
This should be overkill for mounting triangulation.

Looks like my only practical option for lower trailing arms will be a Watts Link on each side connected at the lower arm.

Find it fascinating how many opinions are published on-line regards the lower arm angle.
I currently have them set dead level at normal ride height.
Several sites insist this is correct, others that the drive-shafts should be level, and yet more that the lower link should droop a bit.
I'm going to search for my old XKE manual to get an answer I will trust completely.

Shock mounts are only tacked in place.
Moving them outward would lower the outer end of the arms, bringing them inward raises the ends so long as the shocks remain the same length.

Ideally the driveshafts should be close to level with the lower arms sloping downward from the diff to the hubs. The reason is quite straight forward. when you corner hard the body roll will put positive camber onto the outside wheel, with a sloping lower arm as it is raised in relation to the chassis will create negative camber to cancel out the positive camber from body roll. The opposite is happening to the inside wheel, this does not matter much as the majority of the weight of the car and grip is on the outside wheel.
Too much slope will make the car nervous when driving in a straight line on a bumpy road at speed.
Philip